This model of SPG30 brushed motor comes with 20:1 gearbox attached. The gearbox output shaft is a D-shaped with 15.5mm long and 6mm in diameter.

Mounting

For mounting, the depth of the thread of the motor is 3.2 mm. So, you need to use 3.2 mm length of M3 size screw (not included with the thickness of the bracket). Longer screw will damage the gear box. You may need to consider the thickness of the bracket or other mechanism that you need to attach to the motor for choosing the right length of screw.